创建TabBarController中包含NavigationController

当用TabBarController作为app的根控制器时，还需要在里面的子viewController中加入NavigationController，这种情况在appDelegate可以这样写

MainTabBarViewController *mainTabbar = [[MainTabBarViewController alloc] init];
MessagesViewController *messagesVC = [[MessagesViewController alloc] init];
UINavigationController *messagesNav = [[UINavigationController alloc] initWithRootViewController:messagesVC];
BookingsViewController *bookingsVC = [[BookingsViewController alloc] init];
UINavigationController *bookingsNav = [[UINavigationController alloc] initWithRootViewController:bookingsVC];
mainTabbar.viewControllers = @[messagesNav,bookingsNav];
self.window.rootViewController = mainTabbar;

如果有登录界面，进入后显示TabBarController，可以在登录界面写

MainTabBarViewController *mainTabbar = [[MainTabBarViewController alloc] init];
MessagesViewController *messagesVC = [[MessagesViewController alloc] init];
UINavigationController *messagesNav = [[UINavigationController alloc] initWithRootViewController:messagesVC];
BookingsViewController *bookingsVC = [[BookingsViewController alloc] init];
UINavigationController *bookingsNav = [[UINavigationController alloc] initWithRootViewController:bookingsVC];
mainTabbar.viewControllers = @[messagesNav,bookingsNav];
[self presentViewController:mainTabbar animated:YES completion:nil];

跳转用present，不能用push